Alastair Hay, Professor of environmental toxicology at Leeds University, said osmium tetroxide was a rare catalyst - a chemical that speeds experiments - and could potentially make an explosion occur more rapidly.

But Professor Hay told the BBC it would have to be obtained from a specialist chemical supplier and it did not fit the profile of a typical chemical warfare or dirty bomb agent.

"It would not be in the same category as some radioactive substance which would continue to emit radiation and cause a problem in terms of clean up.

"This would be something present, like a heavy metal like lead, in the environment. I don't think it would be a major hazard and clean up would not be a major problem," he said.

'Psychological terror'

Security expert Dr Sally Lievesly said terrorists were well aware of the psychological impact a chemical bomb could create.

She told BBC One's Ten O'Clock News: "The emergency services would be faced with a terrible scene. They would have to kit out, they'd be delayed and the injuries they'd be seeing would be very bad. So their job is a very difficult one.

"The public, with these types of attack, if the public are not prepared this then becomes a weapon of psychological terror."

On Tuesday night the intelligence services, including MI6, were declining to comment on what appears to be a successful operation by them and the police.

Earlier, a Whitehall official told the BBC that, had this plot succeeded, it would have been the most serious attack on Britain in many years.